Planning to measure an unknown asteroid

Instrument robustness when you don’t know what you’re going to see

We’ve got a leading hypothesis about the asteroid (16) Psyche: it is the metal core of a tiny planet that had its rocky exterior smashed off by some number of hit-and-run collisions, all in the first few millions of years after solids began to form in our solar system, long before the Earth was formed. And now Psyche orbits our Sun in the outer asteroid belt, the biggest bare metal object in our solar system.

Well, we think it’s metal, mostly nickel and iron. Several density estimates of Psyche have been made, including 4,500 ± 1400 kg m^-3 [1], 6,980 ± 580 kg m^-3 [2], 6,490 ± 2,940 kg m^-3 [3,4], and 7,600 ± 3,000 kg m^-3 [5]. These high-density estimates contrast strongly with the estimates for rocky asteroids: 1,380 kg m^-3 to 2,710 kg m^-3 for rocky asteroids, roughly one-third to one-half their parent-rock density of around 3,300 kg m^-3 [6]. (Here I am using “rock” for silicate rock, as opposed to metal.) Most asteroids, therefore, appear to be fractured or have otherwise high porosity, and so their bulk density is lower than their pure material density. And Psyche’s density estimates are higher than all the rocky asteroids.

Psyche’s reflectance spectrum is relatively flat and featureless, consistent with a metal body with about 10% rock on its surface [7].

And, the most definitive measurements of all, metal composition is indicated by a radar albedo (brightness) of 0.42 [8] and a high thermal inertia of 120 J m^-2 S^-0.5 K^-1 [9] , where in comparison the rocky asteroids Ceres, Pallas, Vesta, Lutetia all have thermal inertia from 5 to 30 J m^-2 S^-0.5 K^-1. (Even if the mellifluous units of “J m^-2 S^-0.5 K^-1” don’t feel intuitive to you, it’s easy to see that Psyche, at 120, is really different from the rocky asteroids, at 5 to 30.)

So, we think it’s metal. But we could be completely wrong.

How wrong could we be? And if we are wrong, will the instrument suite we carefully chose for our top hypothesis, the metal core Psyche, still work?

This isn’t a thought process that all missions have to go through these days. We really do know what Mars is made of, and the Moon. And even Europa.

Thus began the process we called “instrument robustness:” how robust is our instrument suite to the many possible Psyches? The basic question is, if instrument X fails, will we still be able to answer Y science objective if Psyche turns out to be Z? This is a pretty large parameter space to investigate, particularly because our second- and third-level hypotheses for what Psyche could be are getting pretty far out there.

I started by examining only our top science objective, Is Psyche a core? To answer this question, we are planning to make six physical measurements of Psyche. All this is explicitly laid out in that brilliant and torturous tool, the Science Traceability Matrix, or STM. Those six physical measurements, shown on the left in Figure 1, require all the instruments (magnetometer, gamma ray and neutron spectrometer [GRNS], imager, plus the gravity experiment carried out with the spacecraft itself as it responds to the asteroid’s varying field during orbit). On the right, I drew symbols of the possible Psyche characteristics.

Figure 1. “Emojis” for the possible Psyches we might encounter, and what we plan to measure. (IFF means “if and only if.”)

The top physical measurement, the magnetic field, could be the key: if Psyche has a significant magnetic field still recorded in its solid body, it was once a core that produced its own dynamo. The magnetic field emojis show, from left to right, a dipole (virtually impossible to exist today), a complex field, and no field.

Next, the existence and scale of rocky material on Psyche’s surface. If all the rocks are intimately mixed below imager resolution, Psyche could be a primitive body that never melted (not a core). If the rocks are in big provinces, then they might be leftover slabs of Psyche’s mantle, or splats from later impacts.

The gravity field and imager will show us if Psyche is a broken-up “rubble pile” or a whole body. This doesn’t determine whether Psyche is a core or not, but it helps interpret other data.

If there are large provinces of rocks, they may be mantle material (achondrites, in the language of meteorites) or primitive materials from later impacts (chondrites). If they are achondrites, probably Psyche is a core and these are parts of its mantle.

Next, there could be provinces of high sulfur (S) and potassium (K) on the surface, expelled there from the solidifying core. This is based on theory, not observation, and we are eager to look for it.

Finally, the all-important nickel content. If Psyche is a core that solidified from the outside in, then the first solids to form are on the surface and they will have the lowest nickel content. Medium nickel content indicates normal core formation, possibly solidifying from the inside out and leaving higher nickel contents on the surface. Very, very low nickel content, however, means Psyche formed at oxidation levels below any yet measured, and is probably primitive material.

Then, I laid out the ten characteristics, each with its emoji, into a matrix. If we made the two measurements that each box in the matrix represents, what would we conclude about Psyche? and in which cases are two measurements not enough to make a conclusion? In Figure 2 those inconclusive cases are shown in pink, and further described in Figure 3.

Figure 2: What conclusions about Psyche would we reach if we had only the two measurements represented by each box? We think some boxes are not physically possible, based on what we know about planetary materials, and they are marked “not expected.” Of course, the lower-left half of the matrix is a duplicate of the top-right half, and so it left blank.

Figure 3. The five cases from Figure 2 where two measurements are not sufficient to conclude whether or not Psyche is a core, and what further measurement is needed. The numbers in the right-hand column refer to the emoji numbers along the top of Figure 2.

We think that three measurements will be enough to break any tie, in case we end up in one of those more difficult pink boxes. Finally, the payload and instrument team members and I talked this over, and we presented it to the larger science and engineering team at a Team Meeting. We are as confident as we can be that we will be able to answer the key question about Psyche, Is it a core? when we arrive in 2026.